The present invention describes a method of preparing p-nitrobenzoic acid by the oxidation of p-nitrotoluene with oxygen or oxygen-containing gases under elevated pressure and temperatures between 80.degree. and 150.degree. C in a saturated aliphatic carboxylic acid as solvent and in the presence of cobalt salts and brominous compounds as catalysts.
Numerous processes are known in which methyl groups on an aryl nucleus are oxidized by means of oxygen in the liquid phase and in the presence of suitable catalysts to form carboxyl groups. Preferred catalysts are cobalt and manganese salts combined with brominous compounds. Whenever solvents appear to be necessary or desirable, it is mainly saturated aliphatic carboxylic acids that are employed for the purpose, preferably acetic acid.
Elevated temperatures are necessary for starting the oxidation reaction and for the achievement of good volume-time yields. Many known processes therefore operate at temperatures above 150.degree. C and in some cases even above 200.degree. C. The optimum reaction conditions depend greatly on the structure and chemical properties of the substance that is to be oxidized. The oxidizability of the methyl group in toluene is thus considerably affected by the introduction of additional substituents in the nucleus.
The nitro group has been shown to cause difficulty in the oxidation (N. Ohta and T. Tezuka, Rept. Govt. Chem. Ind. Research Inst., Tokyo 51, 249-52 (1956); C.A. 51, 281 (1957)). The oxidation of nitrotoluene therefore requires higher reaction temperatures than the oxidation of toluene itself.
On the other hand, in technical oxidation processes low reaction temperatures are desired, for the purpose of suppressing undesired secondary reactions, and if acid solvents are used, the reactor material must satisfy stringent corrosion-resistance requirements. Precisely when acetic acid is used as a solvent, the high-alloy steels often used as vessel material are subject, in the presence of brominous compounds under oxidizing conditions, to corrosion which increases greatly as the temperature increases.
One of the undesirable secondary reactions promoted by high reaction temperatures in the air oxidation of alkyl aromatics is especially the total oxidation of starting product and/or solvent, that is, the oxidative decomposition of these products ultimately to water and carbon dioxide. In the case of acetic acid, for example, in the absence of other oxidizable substances, when air is passed through the solution under elevated pressure, an oxygen absorption is observed beginning at about 150.degree. C and increasing greatly as the temperature rises, which is catalyzed by cobalt acetate, and is simultaneously accompanied by the formation of carbon dioxide and water.
Lastly, when air is passed through an acetic acid reaction solution, increasing amounts of entrained acetic acid vapors are observed as the reaction temperatures increase. The vapor pressure of acetic acid, at about 7.6 atmospheres at 200.degree. C, amounts to three times what it is at 150.degree. C (approx. 2.4 atmospheres). On this basis, at 200.degree. C and at a pressure of 30 atmospheres, the vapor phase will contain approximately 25% acetic acid by volume, as compared with about 8% by volume at 150.degree. C. The separation of the acetic acid from the residual air thus requires a technical expenditure that will be all the greater as the temperature of the reaction solution is higher, for the same working pressure.
From the viewpoints set forth above, the process for the oxidation of p-nitrotoluene according to U.S. Pat. No. 3,030,414, which delivers a yield of 86% p-nitrobenzoic acid at reaction temperatures of 150.degree. to 250.degree. C does not appear to be an optimum one, all the more so because the recommended inclusion of manganese salts as catalysts always results in a greatly discolored reaction product whose discoloration cannot be corrected even by repeated recrystallization from glacial acetic acid. In view of the preferred use of nitrobenzoic acid as a valuable starting product for a number of pharmaceutically useful substances, however, high purity is especially desirable, and a product that is not discolored is demanded.